1. Field of The Invention
The present invention relates to a flame retardant resin composition comprising a thermoplastic resin, a flame retardant and a fluororesin, and a method for producing the same. More particularly, the present invention is concerned with a flame retardant resin composition comprising a thermoplastic resin, a flame retardant and a fluororesin, wherein the fluororesin is present in the form of fibrils, and the thermoplastic resin and the flame retardant are present in the form of a mixture thereof, and wherein, when a breakage-exposed surface of a broken piece of a test sample prepared by injection molding from the resin composition (which breakage-exposed surface of the broken piece is obtained by applying to the test sample a tensile force sufficient to break the test sample) is examined by means of a scanning electron microscope with respect to a predetermined area of 7 .mu.m.times.7 .mu.m wherein the fibrils are dispersed in the entire region of the predetermined area, the dispersed fibrils exhibit a specific dispersion morphology. The flame retardant resin composition of the present invention is advantageous not only in that it has high flame retardancy, especially a high property of dripping-prevention upon being burnt, but also in that a shaped article obtained from the resin composition has excellent appearance, specifically an excellent luster and a decreased occurrence of flow mark.
2. Prior Art
There is an increasing demand for improved safety in the accident of fire. In accordance with this increasing demand, regulations concerning flame retardancy have become rigorous, so that techniques for imparting flame retardancy to resins have become important. Thus, flame retardancy is one of the most essential properties for resins for use in various application fields, especially office automation machines, such as computers, word-processors, printers and copying machines, and household electrical appliances, such as television sets and game machines.
For obtaining an evaluation of high-level flame retardancy in the Vertical Burning Test described in UL-Subject 94 prescribed by Underwriters Laboratories Inc., U.S.A., it is necessary that a resin do not cause a dripping of a flaming particle. Also from a practical viewpoint, for preventing a spread of fire, it is an important task to impart a dripping-preventive property to a resin.
For meeting the demand for resins having a dripping-preventive property, it has been attempted to add an anti-dripping agent to thermoplastic resins. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 3-190958 discloses a thermoplastic resin composition which contains a silicone resin as an anti-dripping agent. Recently, however, it is a general tendency to use, as an anti-dripping agent, a fluororesin having a fibril-forming ability, and many attempts to use such a fluororesin have been made.
In this connection, however, it is noted that, although U.S. Pat. No. 3,005,795 discloses a resin composition comprising a thermoplastic resin and a fluororesin having a fibril-forming ability, the incorporation of a fluororesin having a fibril-forming ability in this U.S. Pat. No. 3,005,795 is not intended to improve the dripping-preventive property of a thermoplastic resin, but intended to increase the melt viscosity of a thermoplastic resin.
As mentioned above, various attempts have been made in which a fluororesin is used in order to improve flame retardancy, especially dripping-preventive property.
For example, Unexamined Japanese Patent Application Laid-Open Specification No. 59-64651 (corresponding to U.S. Pat. No. 4,463,130) discloses a composition obtained by adding a polytetrafluoroethylene (PTFE) to a resin composition comprising a polycarbonate resin, a styrene polymer and a flame retardant. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 63-286463 (corresponding to U.S. Pat. No. 4,786,686) discloses a composition obtained by adding a fluororesin having a fibril-forming ability to a resin composition comprising a polycarbonate resin and a rubber polymer. In these two prior art documents, an improvement in dripping-preventive property is mentioned as an effect which can be obtained by the techniques of these prior art documents; however, in these prior art documents, there is no description as to whether or not there occurs the formation of fluororesin fibrils having a very small diameter, and whether or not there occurs the formation of fluororesin fibrils having a network configuration or a branched configuration, and there is no description as to how to obtain fluororesin fibrils having a very small diameter and having a network configuration or a branched configuration. Moreover, in these two prior art documents, there is no description about a technique in which an improved dripping-preventive property can be efficiently achieved with reproducibility, using a small amount of a fluororesin.
Japanese Patent Application prior-to-examination Publication (kohyo) Nos. 5-504582 (corresponding to U.S. Pat. No. 5,109,044) and 4-506829 (corresponding to U.S. Pat. No. 5,276,078) disclose a composition obtained by adding PTFE (having a fibril-forming ability and having the capability of exhibiting a thermal shrinking ratio of 15% or more) to a resin composition comprising a polycarbonate resin and an ABS resin. In these prior art documents, an improvement in dripping-preventive property is mentioned as an effect which can be obtained by the technique of these prior art documents, and it is described that this effect can be obtained by virtue of the fibrous structure of PTFE and a thermal shrinkage of the fibrous structure. However, in these two prior art documents, there is no description as to whether or not there occurs the formation of fluororesin fibrils having a very small diameter, and whether or not there occurs the formation of fluororesin fibrils having a network configuration or a branched configuration, and there is no description as to how to obtain fluororesin fibrils having a very small diameter and having a network configuration or a branched configuration. Moreover, in these two prior art documents, there is no description about a technique in which an improved dripping-preventive property can be efficiently achieved with reproducibility, using a small amount of a fluororesin.
There have been known several methods for incorporating a fluororesin into a resin composition.
For example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 60-13844 (corresponding to U.S. Pat. No. 4,810,739) and 2-32154 (corresponding to U.S. Pat. No. 5,061,745) disclose a method for incorporating PTFE into a resin composition comprising a polycarbonate, an ABS resin and a flame retardant, wherein an aqueous dispersion of PTFE (having a solids content of about 60% by weight) and an aqueous emulsion of an ABS resin are mixed with each other, and the resultant mixture is subjected to coagulation to obtain a coagulated PTFE/ABS composition, and the obtained coagulated composition is melt-kneaded together with a polycarbonate and a flame retardant. This prior art document describes that this method improves the flame retardancy and dripping-preventive property of a thermoplastic resin composition and solves the surface defect problem of a shaped article.
Unexamined Japanese Patent Application Laid-Open Specification No. 4-272957 (corresponding to EP No. 483510) discloses a method for incorporating an elastomeric, impact resistance-improving agent into a polycarbonate, wherein elastomer particles are coated with a PTFE having a fibril-forming ability, and the resultant PTFE-coated elastomer particles are added to a polycarbonate. This prior art document describes that, by this method, the dispersibility of elastomer particles is improved to thereby prevent a coagulation of elastomer particles, so that a resin composition having improved flame retardancy and having a capability of providing a shaped article having improved appearance can be obtained.
Unexamined Japanese Patent Application Laid-Open Specification No. 8-188653 (corresponding to EP No. 718346) discloses a method for dispersing a solid additive, such as PTFE, into a thermoplastic resin, wherein a mixture of a thermoplastic resin solution and a PTFE dispersion is subjected to, e.g., spray drying to thereby remove the solvent and dispersion medium from the mixture, and the resultant spray dried composition is melt-kneaded. This prior art document describes that, by this method, the dispersibility of a solid additive, such as PTFE, can be improved, so that a shaped article having improved appearance while maintaining a satisfactory mechanical strength can be produced.
By these methods for incorporating a fluororesin into a resin composition, the dispersibility of PTFE in a fibrous form can be improved, so that the appearance of a shaped article can be improved. However, these methods can only achieve an improved dispersibility of a fibrous PTFE. By these methods, the dripping-preventive performance of PTFE cannot be improved, so that a satisfactory dripping-preventive performance of PTFE cannot be achieved. In addition, the improvement in the appearance of a shaped article is unsatisfactory.
In recent years, in the field of office automation machines, the coloring of a shaped article by performing a coating on the shaped article is not frequent, but the coloring of a shaped article is conducted mainly by incorporating a colorant into a molding material to be used for producing the shaped article. Therefore, it is desired to produce a shaped article having a good appearance and a decreased occurrence of flow mark.
A task of the present invention is to provide a flame retardant resin composition comprising a thermoplastic resin, a flame retardant, and a fluororesin as an anti-dripping agent, wherein the fluororesin is present in the form of fibrils having a specific dispersion morphology which enables the resin composition to have advantages not only in that it has high flame retardancy, especially a high property of dripping-prevention upon being burnt, but also in that a shaped article obtained from the resin composition has excellent appearance. Another task of the present invention is to provide a method for producing such a resin composition.